A conventional baby stroller is made to be foldable so as to minimize the space required of storage and carry. A conventional collapsible structure for baby strollers is disclosed in TW Utility Model No. 315186 titled “A Safety Lock Device for a Collapsible Baby Stroller. The applicant discloses a frame which can be folded longitudinally and laterally, and the frame includes an upper frame having two handles on two sides thereof, a lower frame which is pivotally connected to the upper frame by a joint member which has an engaging portion, a first collapsible unit located at two sides of the lower frame and can be expanded and folded, and a safety lock device which is connected between the upper frame and the joint member. The safety lock device is engaged with the engaging portion so that when the first collapsible unit is released, the upper frame and the lower frame maintain their expanded or use status.
However, the conventional collapsible structure is complicated, difficult to be manufactured, and includes high cost. There are problems during operation and the safety lock device can be unintentionally activated by impact so that the conventional collapsible structure cannot achieve its desired function.